Photoconductive layer and method of making the same



'July 2, 1968 EIHACHI SAITO 3, PHOTOCONDUCTIVE LAYER AND METHOD OF MAKING THE SAME I Filed May 25, 1965 InzenTar Eihachi Scu'to United States Patent 3,391,022 PHOTOCONDUCTIVE LAYER AND METHOD OF MAKING THE SAME Eihachi Saito, Tokyo, Japan, assignor to Sony Corporation, Shinagawa-ku, Japan, a corporation of Japan Filed May 25, 1965, er. No. 458,636 5 Claims. (Cl. 117-211) ABSTRACT OF THE DISCLOSURE Method of applying a photoconductive layer to a transparent base which involves rotating the base about a substantially vertical axis, and projecting an evaporated photoconductive material at the base at an acute angle to the vertical to thereby build up a photoconductive layer having a continuous phase of photoconductive material with minute cavities distributed therethrough.

The present invention relates to the manufacture of photoconductive layers used in the target areas of image tubes, for example, in vidicon tubes used as television camera tubes.

Closed circuit television cameras frequently use vidicon tubes because of their small size and simplicity of operation and adjustment. Vidicon tubes have a moderate sensitivity, and can be used in most industrial locations without auxiliary lighting.

The vidicon is a storage type of camera tube whose signal output is developed directly from the target of the tube and is generated by a low velocity scanning beam from an electron gun. The target consists of a transparent signal electrode deposited on the face plate of the tube, and a thin layer of photoconductive material deposited over the electrode. The photoconductive layer serves two purposes. First, it is the light sensitive element, and second, it forms the storage surface for the electrical charge pattern that corresponds to the light image falling on the signal electrode.

The photoconductor has a fairly high resistance when in the dark. Light falling on the material excites additional electrons into a conducting state, lowering the resistance of the photoconductive material at the point of illumination. A position voltage is applied to one side of the photoconductive layer by means of the signal electrode. On the other side, the scanning beam deposits sufficient electrons at low velocity to maintain a zero voltage. In the interval between successive scans of a particular spot, the light lowers the resistance in relation to its intensity. Current then fiows through the surface at this point and the back surface builds up a positive voltage until the beam returns to scan the point. The signal output current is generated when the beam returns this positively charged area to zero voltage. An equal number of electrons flow out of the signal electrode and through a load resistor, developing a signal voltage that is fed directly to a low noise, video signal amplifier.

A fine mesh screen stretched across the tube near the target causes the electron scanning beam to decelerate uniformly at all points and approach the target in a perpendicular manner. The beam is brought to a sharp focus on the target by the longitudinal magnetic field of the focusing coil and the proper voltage for the focusing electrode. The beam is made to scan the target by varying the magnetic fields of the deflecting coils. The photoconductor is chosen to have a low secondary emission ratio and as .a result does not charge positively when subjected to electron bombardment.

One of the objects of the present invention is to provide a target area for vidicon tubes and the like which has excellent sensitivity and residual image characteristics.

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A further object of the invention is to provide a target for vidicon tubes which can be made by a relatively simple manufacturing process.

A further object of the invention is to provide a method for the manufacture of photoconductive bodies particularly suitable for use in vidicon tubes and the like by a process which produces results which are reproducible in succeeding runs.

Other objects, features, and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings in which:

FIGURE 1 is an enlarged cross-sectional view of a photoconductive layer assembly of the type heretofore employed;

FIGURE 2 is an enlarged cross-sectional view of the photoconductive layer produced according to the present invention;

FIGURE 3 is a somewhat schematic view of a type of apparatus which can be used for the application of the photoconductive layer of the present invention; and

FIGURE 4 is a greatly enlarged fragmentary view showing in somewhat schematic manner how the photoconductive layer of the present invention may be applied.

The residual image time in a vidicon tube is given by the following equation:

where k is a constant, R is the resistance of the electron beam path, and C is the electrostatic capacity of the target layer. The electrostatic capacity is determined by the area, the thickness, the dielectric constant, and the like, of the photoconductive material layer. The resistance R of the electron beam path depends upon the structure of the electron gun and the vidicon. Consequently, the residual image time decreases with a decrease in the electrostatic capacity C. In the prior art, the residual image time is decreased by providing, as shown in FIGURE 1, a transparent electrode surface 2 composed of a so-called NESA layer of tin oxide or the like, deposited by evaporation on one side of a glass plate 1. The photoconductive layer 3 is formed of a granular or sponge-like photoconductive material having intergranular spaces 5 between adjacent, discrete particles 4. The apparent dielectric constant of the photoconductive material layer 3 is thereby made small to decrease the electrostatic capacity C.

However, since the individual particles 4 of the spongelike photoconductive layer 3 provide a relatively large surface area, the carriers caused by the irradiation of light rapidly disappear and the sensitivity of the photoconductive layer decreases. In addition, great contact resistance is created by the abutting particles 4 of the photoconductive material, so that the carrier has difficulty in arriving at the electrode 2, and the sensitivity is thereupon rendered quite low.

To overcome the defects of the foregoing, the present invention provides a photoconductive layer which is substantially free from the difiiculties of the type of structure shown in FIGURE 1. In accordance with the present invention, the photoconductive layer includes a large number of minute cavities which are distributed substantially uniformly throughout a continuous phase of a photoconductive layer formed on a base plate, so that the photoconductive layer has low residual image value, and is highly sensitive.

As illustrated in FIGURE 2, the photoconductive layer 3' of the present invention is formed on and bonded to a transparent electrode surface 2 which is adherent to a glass plate 1. The photoconductive layer 3 contains a large number of small cavities 5' distributed fairly uniformly throughout the continuous layer of photoconductive material. Since the photoconductive layer 3 is not a granular layer as heretofore employed, the contact resistance of the layer is very low. Also, the apparent dielectric constant is low due to the presence of the numerous cavities in the photoconductive material layer. Accordingly, the electrostatic capacity C is low, and the residual image time is thereby decreased.

Furthermore, since the photoconductive material layer 3 is continuous, its surface area is small and the number of recombination centers is small, thereby increasing the sensitivity. Consequently, carriers caused by the light arrive more easily at the electrode surface, and the sensitivity is thereby enhanced.

The sensitivity S of a vidicon tube can be expressed by the following equation:

where V is the target voltage,

7 is a constant,

P is the sensitivity of the photoconductive layer,

a is the dielectric constant of the photoconductive layer, T is the frame time (typically 4 sec.),

C is the electrostatic capacity, and

R is the leakage resistance.

In accordance with the present invention, the sensitivity P of the photoconductive layer itself is improved. In order to prevent lowering of the sensitivities S and P, caused by a decrease in the resistivity of the photoconductive layer 3' because of its continuous nature, it is preferred to use a photoconductive material of high resistivity such as an antimony oxysulfide such as Sb OS Sb O S and the like.

In the prior art structure, the resistivity of the photoconductive material layer 3 is determined substantially by the contact between the particles 4. As a result, the resistivity depends upon the granular condition of the photoconductive material, and a photoconductive layer of substantially uniform characteristics is difiicult to obtain. In the photoconductive layer of the present invention, however, the resistivity of the photoconductive material layer 3' depends upon the inherent resistivity of the photoconductive material itself. Consequently, even if conditions in the manufacturing process vary, photoconductive layers of substantially constant resistivity can be obtained uniformly.

In FIGURE 3, there is illustrated an apparatus which can be used for the application of the photoconductive layer of the present invention. A bell jar composed of glass or the like is arranged to be evacuated by an oil diffusion pump 21 coupled to an oil circulation pump 22. An ionization vacuum gauge 23 communicates with the interior of the bell jar 20. Inside the bell jar 20, there is provided a support plate 24 on which there is mounted a motor 25 and a rotary plate 27 is attached to the depending shaft 26 of the motor 25. To the underside of the rotary plate 27 is attached a transparent base plate 1 of glass or the like having deposited thereon a transparent electrode layer 2. A shutter 28 is disposed immediately below the surface to be coated. A thermistor temperature control unit 29 is attached to the underside of the support base 24 in proximity to the rotating plate 27. A heater 30 is mounted on a base 31 of the bell jar 20, and serves to heat the transparent base 1 to a suitable temperature prior to deposition by evaporation. Normally, this temperature is in the range from about 50 C. to 200 C. In the lower portion of the bell jar, there is provided an evaporating deposition source 32, and a mesh-like guide 33 is placed between the source 32 and the transparent base plate 1 being coated. Accordingly, the direction in which the material is deposited by evaporation on the rotating plate from the source 32 is adjustable to an angle 0 with respect to the vertical axis of the shaft 26.

In the device shown in FIGURE 3, the transparent base plate 1 is heated by the heater 30 prior to deposition by evaporation, after which the shutter 28 is opened, and the material is deposited on the rotating plate. When the material to be deposited has reached a predetermined thickness, generally from 0.5 to 3 microns, the shutter is closed and the deposition is completed. In one such deposition, the bell jar was evacuated to about 3X10- millimeters of mercury, and the deposition was carried out for about 60 minutes. The rotational speed of the rotary plate 27 was 18 rpm, and the temperature of the transparent base plate was raised to a value of 50 C. to 200 C. prior to evaporation.

The angle 8 was in the range from 15 to and this was most conveniently held at 45.

The results of measurements of vidicon tubes produced with the preferred materials of the present invention, and by conventional means are given in the following table:

Residual Thickness Sensitivity Image of Deposi- Material tion 4 2 4 N0. 2 140 1.2 {Sb203+29%$b 275 0. a 260 1. 8 SbzSa+l9%Sb 62 90 2. 4 Sb2Sa+l0%Sb 100 100 1. 2 SbzSa In the above table, No. 6 was a conventional vidicon tube. The values of sensitivity and residual image are unitless, and are given in relation to values of 100 for vidicon tube No. 6.

The schematic method for the deposition of the photoconductive layer on a stationary plate is illustrated in FIGURE 4 of the drawings. The glass plate 1 having the transparent electrode 2 is held in a stationary position, and subjected to evaporation in a vacuum from a source which deposits the evaporated material onto the electrode 2 along a direction identified at reference numeral 11. This deposition provides a layer having projections 8' extending toward the source of the material. Next, the deposition is carried out from the righthand corner, as illustrated by the directon lines 12 to provide projections 8" extending toward the source of the deposited material. The consolidation of the two deposits from opposite directions results in the formation of minute cavities 5' between the projections 8' and 8".

The sensitivity of the photoconductive layer produced according to the present invention is about 1.5 to 2.5 times higher than that of layers prepared by conventional processes. The residual image value is not impaired substantially by the improvement in sensitivity.

Furthermore, the deposition by evaporation can be done in a single step, whereas in the past, the deposition had to be carried out 'by a vacuum treatment, followed by a deposition in an inert gas such as argon gas.

It will be evident that various modifications and variations can be effected without departing from the scope of the novel concept of the present invention.

I claim as my invention:

1. A method of applying a photoconductive layer to a transparent base having a transparent electrode layer thereon which comprises rotating said base along a substantially vertical axis, and projecting an evaporated photoconductive material at said base at an acute angle to the vertical to thereby build up a photoconductive layer having a continuous phase of photoconductive material with minute cavities distributed therethrough.

2. The method of claim 1 in which said transparent base is preheated to a temperature of from about 50 C. to about 200 C. prior to deposition of said photoconductive layer.

3. The method of claim 1 in which the photocond-uctive cavities distributed therethrough, said photoconductive layer is deposited under under substantially vacuum conlayer being applied by the method of claim 1. ditions.

4. The method of claim 1 in which said photoconduc- References Cited tive layer includes an oxysulfide of antimony. 5

5. A photoconductive body comprising a transparent UNITED STATES PATENTS base, a transparent electrode layer afiixed to said base, 2,759,861 8/1956 Collins et al. 148-15 and a photoconductive layer attached to said electrode layer, said photoconductive layer consisting of a con- WILLIAM L. JARVIS, Primary Examiner. tinuous phase of photoconductive material with minute 10 

